1. Field of the Invention
The present invention relates to a lumped-element diplexer used in the field of communications, and more particularly, to a lumped-element diplexer realized in a multi-layered substrate.
2. Description of the Prior Art
If any one trend has been constant throughout the development of mobile phone handsets, it's that they have continued to become more compact while offering improvements in both functionality and utility. In order to progress from the veritable house-bricks of the nineteen eighties, the components used in communication systems design have had to shrink in terms of both physical size and power consumption. Surface mount technology and advances in microchip, power cell and display design have all played their part, nevertheless, manufacturers continue to place emphasis on, and devote considerable resources to, the miniaturization of each and every component required to make a mobile phone handset.
Diplexers are widely used as building blocks in communication systems design. A diplexer is a three terminal device for separating signals of different frequencies or frequency bands, to different ports. They are commonly used in multi-band communication systems as band-separators (please see FIG. 1). For example, in mobile phone handsets designed to work under the GSM900/DCS1800 dual-band system, a diplexer 100 is usually adopted to connect the common dual-band antenna 101 and the relevant Radio Frequency (RF) circuits 102 & 103 for the GSM900 and DCS1800 frequency bands.
Diplexer performance is characterized in the following terms: insertion loss, guard band, isolation, and out-of-band rejection. Insertion loss is generally taken to mean the negative effect that the diplexer itself has on the signal it is processing, in terms of signal strength and loss thereof. The origins of this term are in transmission line theory and insertion loss is generally expressed in decibels (dB). The term guard band, in this application at least, refers to the bandwidth that lies between the frequency bands of interest. For example, in the case of the GSM900/DCS1800 dual-band mobile phone system, the guard band will be the band of frequencies between 960 MHz and 1710 MHz. The term isolation can be regarded as the devices ability to separate frequency bands cleanly, i.e. without the occurrence of internal cross-talk leading to output of mixed signals at the output ports. Out-of-band rejection, as the name suggests, is the ability of the diplexer to reject signals at frequencies outside of the frequency bands of interest.
Generally implemented using some of the common building blocks of RF signal conditioning, such as high-pass, low-pass, band-stop and band-pass filters, many configurations of diplexers have been proposed; they can be implemented using two band-stop filters, two band-pass filters, or one low-pass and one high-pass filter. For applications where the two frequency bands to be separated are not close, such as the GSM900/1800 mobile phone application, the low-pass/high-pass filter combination is preferable for its ease of implementation, lower insertion loss, and good rejection performance. Further improvement of diplexer rejection performance can be accomplished by introducing additional band-stop or notch filters after the low-pass and high-pass filter, as proposed by U.S. Pat. No. 5,880,649, which is included herein by reference.
In order to minimize circuit size, diplexers can be realized as lumped-elements formed in a multi-layered substrate. FIGS. 2 thru 9 show elements typically found in diplexers, these being such reactive components as capacitors and inductors, implemented a multi-layered substrate of the prior art. The layers of FIGS. 2 thru 9 are layered by the numerical order of the figure numbers. Here, the inductors are realized by ‘meandering’ metal strips (items 201, 501 & 801 being examples) while capacitors are formed between metal plates (items 302, 702, 703 and 802 being examples) on different layers. Significantly, in the implementation shown, the capacitive and inductive elements are separated by ground planes (see FIGS. 4, 6 & 9), which are included to reduce the effects of parasitic mutual couplings between the elements. However, this arrangement brings its own problems in that, not only does the inclusion of ground planes take up precious space, but it also introduces the problem of parasitic capacitance occurring between the reactive elements and the ground planes themselves. In addition, on several layers of the prior art diplexer layout there are more than one circuit component (FIG. 2, 7 & 8 refer), this will also increase the required circuit area.
With the advent of so called third generation or 3G technologies, there seems to be no immediate possibility of discontinued demand for miniaturized components, on the contrary, there is every indication that the communications industry will continue to demand improved performance from smaller and smaller packages for some time to come. Hence, there is a clear need for the development of increasingly compact diplexers, along with other communications components.